This is a Gnu extension for private keys with stripped secrets, which
can be created by GnuPG's --export-secret-subkeys mode.
Before this patch, "ERROR: unknown s2k type! 101" would be logged as an
error (though other than that, things seemed to function normally).
Signature checks were failing, in the binary-document case of
openpgp_packet_signature.verify() the variable i was being referenced
when there was no i in scope; instead it was getting 8 from a global
variable i, and trimming 8 bytes off the beginning of the data.
The global i was being set by the CAST5 cipher -- this also fixes
that to be a local variable.
Also add unit test to confirm.
